The present invention relates to a slurry for chemical mechanical polishing used in fabrication of a semiconductor device, and more particularly to a slurry for chemical mechanical polishing well suited to use in formation of a buried metal interconnection wherein a tantalum-based metal is utilized as a material for a barrier metal film.
In the formation of a semiconductor integrated circuit such as an ULSI (Ultra Large Scale Integrated circuit) for which progress to attain further miniaturization and more densely spaced arrangement has been, in recent years, gathering more speed, copper has been attracting strong attention as a particularly useful material for the electric connection due to its excellent electromagnetic resistance and considerably low electrical resistance.
A copper interconnection is currently formed, due to problems such as a difficulty to make patterning through dry etching, in the following way. That is, after a sunken section such as a trench or a connection hole is formed in an insulating film and a barrier metal film is formed thereon, a copper film is grown by the plating method so as to fill up the sunken section, and then by conducting the chemical mechanical polishing (referred to as xe2x80x9cCMPxe2x80x9d hereinafter) until the surface of the insulating film other than the sunken section is completely exposed, the surface is planarized, and thereby formation of electric connection sections such as a,buried copper interconnection which is substantially made of copper. filling the sunken section, a via plug, a contact plug and the like are accomplished.
Now, with a reference to FIG. 1, a method of forming a buried copper interconnection is described below.
Firstly, on a silicon substrate where a semiconductor device is formed (not shown in the drawing), there is formed a lower interconnection layer 1 made of an insulating film having a lower interconnection (not shown in the drawing). Thereon, a silicon nitride film 2 and a silicon oxide film 3 are formed consecutively in this order, as shown in FIG. 1(a), and then in the silicon oxide film 3 a sunken section in the form of an interconnection pattern is formed to reach the silicon nitride film 2.
Next, as shown in FIG. 1(b), a barrier metal film 4 is formed by the sputtering method. On the film, a copper film 5 is applied to the entire surface by the plating method so as to fill up the sunken section.
After that, as shown in FIG. 1(c), the copper film 5 is polished by means of CMP to planarize the substrate, surface. Polishing by the CMP continues until the metal over the silicon oxide film 3 is completely removed, as shown in FIG. 1(d).
In such a formation of a buried copper interconnection as described above, a barrier metal film is formed as a base film, for the purpose of preventing diffusion of copper into the insulating film and the like. However, when a tantalum-based metal such as Ta or TaN is employed as a barrier metal film, there may arise a problem that with a conventional polishing slurry the polishing rate for the barrier metal film made of Ta or TaN becomes smaller than that for the copper film, owing to the substantially high chemical stability of Ta and TaN. Specifically, when the formation of a buried copper interconnection or such is carried out using the CMP with a conventional polishing slurry, a considerable difference between rates for the copper film and the barrier metal film are produced, which may bring about dishing and erosion.
Dishing is a phenomenon that copper in the sunken section is excessively polished so that the center of the copper film in the sunken section falls back with respect to the plane of the insulating film laid on the substrate, as shown in FIG. 2. A conventional polishing slurry requires an ample polishing time to remove the barrier metal film 4 lying on the insulating film (silicon oxide film 3) thoroughly because of the considerably low polishing rate for the barrier metal film. The polishing rate for the copper film 5 is, however, higher than that for the barrier metal film 4, so that the copper film 5 becomes excessively polished to create dishing.
Erosion is, on the other hand, a phenomenon that polishing in a densely-spaced interconnection region proceeds excessively, compared with that in a region with a low interconnection density such as an isolated interconnection region, so that the surface of the densely-spaced interconnection region falls back with respect to the surface of other regions, as shown in FIG. 1(d). When, in a wafer, the densely-spaced interconnection region where many buried sections formed of the copper film 5 are present is considerably separated from the isolated interconnection region where a few buried section formed of the copper film 5 are present by, for example, a region without any interconnections, and the copper film 5 is polished faster than the barrier metal film 4 or a silicon oxide film 3 (an insulating film), then, in the densely-spaced interconnection region, a polishing pad pressure to the barrier metal film 4 or the silicon oxide film 3 becomes higher than that in the isolated interconnection region. As a result, in the CMP step after exposing the barrier metal film 4 (the step of FIG. 1(c) and thereafter), there is produced a difference in polishing rate by the CMP between the densely-spaced interconnection region and the isolated interconnection region so that the insulating film in the densely-spaced interconnection region is excessively polished to create erosion.
Dishing created in the step of forming an electric connection section in a semiconductor device as described. above may cause an increase in interconnection resistance and connection resistance as well as an increase in electron migration liability, which may lower the reliability of the device. The creation of erosion may also adversely affect the evenness of the substrate surface, the effect of which becomes more prominent in a multi-layered structure so that problems such as an increase and a variation in interconnection resistance may arise.
So far, various investigations have been made to overcome these problems.
For example, in JP-A 83780/1996, it is disclosed that dishing in the CMP step may be prevented by using a polishing slurry which contains benzotriazole or its derivative and thereby forming a protective film on a copper surface.
Further, in JP-A 238709/1999, there is a similar description of a prevention effect that triazole compounds have on dishing.
Further, in JP-A 163141/1998, there is disclosed a polishing composition for a copper film, which comprises a polishing material and water, further containing an iron (III) compound dissolved in the composition. In Examples therein, it is further described that, using colloidal silica as the polishing material and iron (III) citrate, ammonium iron (III) citrate or ammonium iron (III) oxalate as an iron (III) compound, the polishing composition can increase the polishing rate for a copper film and prevent surface defects such as dishing and scratches from occurring.
However, these publications describe nothing concerning polishing of the barrier metal film made of a tantalum-based metal or erosion.
Meanwhile, with the object of improving efficiency of the polishing slurry, addition of amine compounds has been the subject of investigation.
For example, in Examples of JP-A 44047/1998, it is described that, when CMP is carried out using a polishing slurry that contains an alumina polishing material, ammonium persulfate (an oxidizing agent) and a specific carboxylic acid, a difference in polishing rate between an aluminium layer for interconnection and a silicon oxide film becomes widened and, at the same time, a removal rate of a titanium film that serves as a barrier metal film becomes heightened.
Further, in JP-A 46140/1998, there is disclosed a composition for chemical mechanical polishing which contains a specific carboxylic acid, an oxidizing agent and water, whose pH is adjusted to 5-9 using ammonia as an alkali.
Further, in JP-A 21546/1999, there is disclosed a slurry for chemical mechanical polishing which contains urea, a metal oxide polishing material and ammonium oxalate. In Examples therein, there is further described an example wherein Cu, Ta and PTEOS were polished using a slurry with a pH value of 7.5, which was prepared using alumina as the polishing material, hydrogen peroxide as an oxidizing agent, benzotriazole as a film-forming agent and tartaric acid or ammonium oxalate as a complex-forming agent.
In addition, in JP-A 501771/2000, there is described an addition of a hydroxylamine compound or a hydroxylamine salt as a selective redox compound. Therein, an addition of an ammonium peroxy compound is also described.
Further, in JP-A 302633/1999, an addition of a water-soluble amine into a polishing slurry is described. Examples of a water-soluble amine mentioned therein include alkanolamines such as diethanolamine, triethanolamine, 2-aminoethanol, and aminoethylethanolamine; polyamines such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, and triethylenediamine; and imines such as polyethyleneimine.
As set forth above, these publications disclose the use of amine-based compounds. Nevertheless, none of them describe a use of a higher-mono-primary amine with a characteristic structure as an amine-based compound. Moreover, even if any polishing slurry disclosed in those publications is utilized, dishing and erosion in a buried copper interconnection cannot necessarily suppressed satisfactorily.
Further, in JP-A 204474/1999, an addition of an organic amine into a polishing slurry is described. Examples of an organic amine mentioned therein include lower-mono-primary amines such as methylamine, ethylamine and isopropylamine; secondary amines such as dimethylamine, diethylamine and diisopropylamine; tertiary amines such as trimethylamine, triethylamine and triisopropylamine; and aromatic amines such as aniline. Nevertheless, a matter of the primary concern in that publication is polishing of a fluorine-containing film so that an organic amine is utilized merely as a substitute for potassium hydroxide, and a particular use of a mono-primary amine with a higher substituent as an organic amine is not reported. Neither is suggested that the use of a higher-mono-primary amine can reduce the polishing rate of the tantalum-based barrier metal film and, therefore, suppress the occurrence of dishing and erosion in the buried copper interconnection section satisfactorily.
As described above, although a number of proposals for adding an amine-based compound have been made, no polishing slurry with a higher-mono-primary amine is reported. Because of this, the polishing rate for a tantalum-based barrier metal film is, in some cases, reduced and, consequently, the occurrence of neither dishing nor erosion in the buried copper interconnection section is satisfactorily suppressed.
Accordingly, an object of the present invention is to provide a slurry for chemical mechanical polishing, which can suppress the occurrence of dishing and erosion in the CMP and form a buried interconnection with a little variation in interconnection resistance when forming a buried copper interconnection in which a tantalum-based metal is utilized as a barrier metal film.
The present invention relates to a slurry for chemical mechanical polishing to polish a copper-based metal film formed on a tantalum-based metal film, which comprises a polishing grain, an oxidizing agent and a higher-mono-primary amine.
In the present invention, a copper-based metal refers to copper or an alloy the main component of which is copper, and a tantalum-based metal, tantalum (Ta) or tantalum nitride (TaN).
The polishing slurry of the present invention can reduce the polishing rate for a tantalum-based metal film and, thus, increase the difference of the polishing rates between the tantalum-based metal film and a copper-based metal film so that the function of the tantalum-based metal film as a stopper film (polishing stopper) in polishing of a copper-based metal film is enhanced. As a result, in formation of a buried interconnection of a copper-based metal with a barrier metal film made of a tantalum-based metal, dishing and erosion which may result from CMP can be prevented from occurring and a buried interconnection of a copper-based metal wherein a variation in interconnection resistance is well suppressed can be formed.